U.S. Pat. No. 5,119,774, entitled "Direct Acting Hydraulic Valve Lifter" issued on Jun. 9, 1992 by Krieg et al., describes a direct acting hydraulic valve lifter (DAHVL) having various features which individually and/or in combination may provide reduced reciprocating mass with lower oil loss in operation, faster filling of the lifter after draining and more positive discharge of air from the lifter. Vent means from the oil chambers between the hydraulic element assembly and the follower cylinder supporting it provide for the passage of air within the DAHVL.
U.S. Pat. No. 5,129,374, entitled "Hydraulic Tappet", issued on Jul. 14, 1992 by Flavio, provides structure within the tappet which causes the oil to traverse a 360.degree. circular path and through two vertical levels in flowing from the inlet to the inner reservoir within the tappet. In this manner, any air bubbles stay in the tappet assembly but do not enter into the oil reservoir in the high pressure chamber.
U.S. Pat. No. 4,615,306, entitled "Engine Valve Timing Control System" issued on Oct. 7, 1986 by Wakeman and assigned to a common assignee, describes a system using electrohydraulic valve lifters operatively connected to an ECU to provide real time changes in engine valve timing. Pressure pulses within the system, operate to "home" the lifters to the base circle of the timing cams. The ECU controls a solenoid which controls the passage of fluid to and from the lifters.
U.S. Pat. No. 4,671,221, entitled "Valve Control Arrangement, issued on Jun. 9, 1987, by Geringer et al. describes a DAHVL wherein the in-line height of the DAHVL requires the full opening height of the valve to be the lobe height of the cam. In addition this patent shows valve damping by means of a "so-called valve brake" at the end of the closing travel of the engine valve. This brake comprises a ring shaped chamber 54 which is increasing closed by means of a gap between a projection of the housing block and a ramped shaped annular chamber. As the chamber narrows with increasing overlapping of the ramps and the face of the ring-shaped projection the fluid is squeezed until it can flow out of a smaller gap. As is well known this brake is sensitive to the viscosity of the hydraulic fluid.
U.S. Pat. No. 4,674,451, entitled "Valve Control Arrangement for Internal Combustion Engines with Reciprocating Pistons" issued on Jun. 23, 1987 by Rembold et al., describes a DAHVL having the full opening height of the valve to be the lobe height of the cam. The DAHVL is comprised of two pistons separated by a chamber which is controlled by an electromagnetically controlled directional control valve. Damping of the pistons is not a concern.
U.S. Pat. No. 4,696,265, entitled "Device for Varying a Valve Timing and Lift for an Internal Combustion Engine" issued on Sep. 29, 1987 by Nohira, describes a DAHVL used in an electronic valve timing system. Damping of the pistons and hence the engine valve is not a concern.
U.S. Pat. No. 4,796,573, entitled "Hydraulic Engine Valve Lifter Assembly does not show bucket tappets. Again there is shown in-line pistons, in this case three, wherein a damping chamber is between a cam follower piston and a valve damping piston. An separate last adjuster piston is coaxial with respect to the previous two pistons. Damping is performed by reducing the size of the flow passages between the chambers as the pistons near their home position.
U.S. Pat. No. 5,005,540, entitled "Valve Timing Control System for an Internal Combustion Engine", issued on Apr. 9, 1991 by Watanabe, teaches a DAHVL utilizing two pistons, which appear to be similar to bucket pistons but no so described, and the ratio of their respective areas to multiply the lift of the timing cam to open the engine valve the desired amount. Damping of either of the pistons is not discussed.
U.S. Pat. No. 5,158,048 entitled "Lost Motion Actuator" by Robnett et al., and assigned to a common assignee describes a dual, first and second, piston hydraulic actuator having an orifice disk with a thin edge orifice to control damping of the second piston.